1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, and so on, more specifically to an image forming apparatus having a convey belt for conveying a recording material to a transfer position.
2. Related Background Art
FIG. 7 shows a color image forming apparatus of four drum type, which is an example of the image forming apparatus having a convey belt for conveying a recording medium to the transfer position where a developer image formed on an image bearing body is transferred onto the recording medium. In order to achieve speedup, the color image forming apparatus of four drum type has four image forming units. Each image forming unit comprises an image bearing body and process means provided around the image bearing body. Images which are formed in respective image forming units and borne on respective image bearing bodies are transferred, in sequence, onto a recording medium, which is supported and conveyed by a convey means moving in the vicinity of the image bearing bodies.
In the image forming apparatus shown in FIG. 7, the image forming units Pa, Pb, Pc and Pd required to form a full-color image comprise respectively photosensitive drums 51a, 51b, 51c and 51d serving as image bearing bodies, and, around these photosensitive drums, chargers (charger means) 52a, 52b, 52c and 52d, optical scanning devices (optical scanning means, not shown), developer devices (developer means) 53a, 53b, 53c and 53d, and cleaners (cleaning means) 54a, 54b, 54c and 54d. Also, these image forming units have, in common, an endless transfer belt 55. Reference numerals 56a, 56b, 56c and 56d denote transfer charger devices for transferring color images formed on the photosensitive drums 51a, 51b, 51c and 51d of respective image forming units onto the recording medium 57 which is supported on and conveyed by the transfer belt 55. The recording medium 57 is supplied from a sheet supply cassette 58, and after the transfer process separated from the transfer belt 55. Subsequently, the images are fixed by a fixing device 59, and the recording medium 55 is discharged onto a discharge tray 60.
The cleaners 54a, 54b, 54c and 54d have their respective cleaning blades 61a, 61b, 61c and 61d made of elastic material such as polyurethane rubber and remove residual toner on the photosensitive drums 51a, 51b, 51c and 51d. Cleaners of this type have been widely used because of their simple construction and small size.
However, toner may splash from longitudinal ends of the cleaning blades 61a, 61b, 61c and 61d of the cleaners 54a, 54b, 54c and 54d, probably because the toner which is not scraped from the image bearing body by the cleaning blade to remain between the cleaning blade and the image bearing body moves, while intercepted by the cleaning blade, toward the longitudinal ends of the cleaning blade as the image bearing body rotates.
Therefore, in order to prevent the toner from splashing from the ends of the cleaning blade, the ends of the blade are sealed by sealing members of felt, urethane foam, or the like. In this case, however, if the sealing members are pressed against the ends of the blade very hard in order to completely prevent toner splash, the blade receives excessive stress and is distorted. Then, the contact pressure of the blade against the image bearing body becomes nonuniform along the longitudinal direction (that is, the direction of the axis of the image bearing body), and the toner on the image bearing body can not be cleaned properly. Accordingly, it is difficult to completely seal the ends of the blade with the sealing members. Especially in an apparatus which reciprocates its cleaning blade in the direction of the rotation shaft of the photosensitive drum when the photosensitive drum is rotated, gaps easily occur between the ends of the blade and the sealing members. Moreover, more toner is pushed toward the ends of the blade by the reciprocating motion of the blade. Accordingly, more toner splashes through the gaps between the blade and the sealing member.
As the toner is intercepted halfway at the ends of the blade, much of it falls vertically (downwards) as lumps. If the transfer belt 55 is arranged right under the cleaners 54a, 54b, 54c and 54d, as shown in FIG. 7, the toner may fall on the transfer belt 55 and blot it. Once the toner blots the transfer belt 55, other parts in the apparatus may be blotted too by the toner carried by the belt, and functions of the apparatus may be deteriorated.
In order to remove the toner on the transfer belt 55, a cleaner 62 may be provided, as shown in FIG. 7. However, when the transfer belt 55 receives the load from the cleaner 62 for cleaning the transfer belt 55, the transfer belt 55 generates small irregular vibration. So, especially at the time of the transfer operation, the toner images on the photosensitive drum can not be transferred accurately onto the recording medium, which may deteriorate the quality of the resultant image.